This invention relates to pneumatic tires and to fabrics for use in building pneumatic tires.
At the present time most pneumatic tires for passenger vehicles are of the radial ply type as described below and fall into three classes namely SR, HR and VR which are tested to operate at speeds up to 180, 210 and over 210 kilometers per hour, respectively. Radial ply type passenger tires are in the main manufactured from a single or multiply carcass of substantially U-shaped section having metal beads at the inner perimeters of the walls. In such a radial ply tire construction the carcass is formed from a rubberised woven fabric having relatively inextensible reinforcing cords running transversely, i.e. radially, from bead to bead. A carcass constructed in this manner requires to be supported in the area of the tread. Commonly this support is provided by two or more steel cord belt plies extending around the outer periphery of the carcass and across the width of the tread. Such steel belt plies are relatively inextensible in the intended direction of rotation of the tire and include steel belt cords disposed parallel to that direction or at a low angle thereto. The belt plies act to restrict the movement of the tread and give the tire better road-holding properties.
In view of the severe speed tests mentioned above, technical difficulties have been encountered in incorporating steel belts into the tread area of the tire. One of the difficulties lies in ensuring good adhesion between the steel and the rubber. When first introduced such steel belted tires were susceptible to tread chunking at high speed due to temperature build-up causing the rubber to degrade to an extent which impaired the adhesion between the steel belt and the rubber tread. This breakdown was particularly prevalent at the edges of the steel belt where excessive temperature build-up takes place, leading to progressive adhesion failure and irregular and severe tread wear in the shoulder area of the tire.
Two solutions to the aforementioned problem have been proposed. The first solution comprises forming a secondary carcass over the steel belt plies using one or two plies of calendered nylon tire cord extending circumferentially around the tire and across the surface of the steel belt and beyond the edges thereof. The tire cord is disposed with its relatively inextensible warp running around the perimeter of the steel belt in the intended direction of rotation of the tire. The addition of this tire cord is commonly known as a "cap-ply", "overlay" or "nylon cording". One example of this solution is disclosed in U.S. Pat. No. 4,284,117.
The second solution comprises utilising two or more steel belt plies in which the edge of each ply is folded upon itself.
Both of the aforementioned solutions are expensive in that they utilise relatively large amounts of either calendered nylon cord or steel cord and, in the second solution, tire building problems can occur both in the folding of the steel belts and in the accurate positioning of each ply of the belt. Furthermore, both solutions inevitably result in a relatively heavy construction.